1. Field of the Invention
This invention relates to a method for making a steel having high tensile strength and good toughness with excellent workability in cold working, the tensile strength being not less than 50 kg/mm.sup.2.
2. Description of the Prior Art
High tensile steels with a tensile strength of 50-100 kg/mm.sup.2 are widely used. For use in construction and shipbuilding, for instance, they are required to have good weldability. For those to be worked to specified dimensions followed by welding as necessary such as in the field of industrial machines or in the case of materials for making pipes or bolts, excellent cold workability and weldability are required. So far, in making high tensile steels of the nonheat treated type or the heat treated (quenched and tempered) type, large amounts of additive elements are used to enhance mechanical properties but inevitably lead to an increased carbon equivalent which is undesirable to weldability. Making high strength steels in this way has a substantial problem in that it is accompanied by an increase in yield ratio and a decrease in toughness and in ductility. Thus, such high strength steels are subject to cracking during cold working due to insufficiency in toughness and ductility or lead to inadequate shaping due to springback resulting from high yield ratios. In the case of nonheat treated high strength materials for bolts or high strength steels for cold forging, working tools have to bear an increased burden during cold working, such as bolt forming or other various kinds of cold forming, whereby the life of the tools is greatly shortened.